Variability in Plucking Thresholds Governs Knick Zone Formation, Shape, and Rate of Migration
Abstract
Bedrock rivers communicate climatic and tectonic signals across landscapes by responding to changes in baselevel. Steep bedrock channels often self-organize into a series of steps and flats collectively called a knick zone. In highly fractured bedrock, fracture-bound blocks comprise the edges of the steps. Plucking of blocks from these ledges results in upstream migration of steps within the knick zone. The rate of step migration is governed by the threshold of motion of individual blocks, which in turn reflects block geometry and flow conditions. We model bedrock channel plucking by explicitly tracking the susceptibility of all blocks to both sliding and toppling. Block plucking is represented as a Poisson process in which waiting times for each block are cataloged, while acknowledging the distribution of low pressures on the downstream side of the block (Hurst et al., 2021). By varying the joint spacing and lithology (abradability) of the bedrock, we explore what governs the heights, migration rates, and locations of the steps. We find that despite steady baselevel lowering, an autogenic response of the system occurs in the form of waves of erosion. In our simplest models, the timing of these pulses of erosion is set by the baselevel lowering rate and the height of the most downstream block. In the presence of a layer of larger blocks, changes in the baselevel lowering rate are not relayed upstream of the resistant layer until the blocks in this layer are plucked. The reach with large blocks eventually achieves a steeper slope. This model embraces the length scales at which the relevant physics operate, and thus represents an important first step toward exploration of the controls on geometry and erosion rate of steep bedrock channels.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2021
- Bibcode:
- 2021AGUFMEP41B..04H